Generally, large generators are filled with pressurized hydrogen to cool the generator. The purity of the hydrogen dictates its effectiveness in cooling the generator; the higher the purity, the more efficiently the hydrogen cools the generator. Typically, the hydrogen has purity levels in excess of 95%. When purity levels fall below this level of purity, the cooling characteristics of the hydrogen are significantly reduced.
A significant source of contaminants in the hydrogen stream is oil which enters the hydrogen from the lubrication oil and exists in the hydrogen in the form of small droplets and gaseous molecules. Air is often trapped in the oil droplets and is thereby also suspended in the hydrogen stream. Impurities may also comprise styrene, anomine, and various other gases which emanate from the epoxies which are used throughout the generator. Solid particulates generated by the heating of non-metallic components might also be suspended in the hydrogen.
In addition to adversely effecting the cooling qualities of the hydrogen, impurities can adversely effect the electrical quality of the insulating components of the generator. Also, these impurities increase the density of the mixture of gases in the generator and thereby add to windage loss. Further, it should be noted that hydrogen has a greater tendency to explode when found in purity levels of 4 to 74%; thus, lower purity levels increase the possibility of explosion.
A significant and sometimes primary cause for the existence of oil mist and air within the cooling hydrogen of a turbine generator is the leakage of oil-laden gas from the generator's defoaming tanks into the generator's stator housing through the generator rotor's labyrinth seals. Many types of electrical generators utilize gland seals to contain hydrogen within the generator's frame. These gland seals operate by surrounding a portion of the generator rotor, with a very small gap between the stationary gland seal and the rotating rotor, and injecting a stream of oil in the interface therebetween. The passage of oil in an inboard axial direction prevents hydrogen gas from escaping from within the generator frame through this interface. Upon leaving this interface region of the gland seal, the oil is collected in a defoaming tank and recirculated within the oil system of the generator. These gland seals are located at both the turbine and exciter end of the rotor shaft and are provided with a deflector which prevents the oil from splashing directly against the labyrinth seals as it is ejected from the gland seals. Although the deflectors are generally successful in this function, it is possible that a quantity of oil can be ejected from the gland seals with sufficient velocity to enter the labyrinth seals.
A more severe cause of oil contamination of the hydrogen coolant is the defoaming tank itself. The defoaming tank contains oil in both liquid and vapor form, i.e. oil particles suspended in the hydrogen gas. The defoaming tank contains a quantity of liquid oil at its bottom portion which will eventually be recirculated through the generator's oil system. Above this liquid oil is a mixture of hydrogen gas with oil mist dispersed throughout it and with gaseous oil molecules mixed therethrough. This contaminated gas within the defoaming tank is separated from the cooling hydrogen which exists within the generator's frame by the above-mentioned labyrinth seals. However, if the pressure within the defoaming tank exceeds that of the hydrogen gas on the opposite side of the labyrinth seal, the contaminated gas from above the oil in the defoaming tank can flow through the labyrinth seal into the cooling region of the generator frame and contaminate the much purer quantity of hydrogen used to cool the generator.
Although the pressure of the defoaming tank is intended to be kept at a value less than that of the hydrogen within the generator frame. various factors, including a rise of the oil temperature within the defoaming tank, can cause the pressure within the defoaming tank to exceed that of the hydrogen gas located within the generator. As described above, this increase in gas pressure within the defoaming tank can trigger the harmful flow of contaminated gas through the labyrinth seal and into the generator frame causing contamination of the generator's hydrogen cooling system.
Various methods have been developed to address the problems associated with impurities in the cooling hydrogen stream. Some have proposed vacuum treating the sealing oil to remove impurities. In such systems, a vacuum maintains hydrogen purity by removing gases including hydrogen from the oil prior to the oil entering the seal ring. The major drawback of such systems is the amount of hydrogen that is consumed. When the de-gassed oil enters the seal ring and contacts the hydrogen atmosphere, the oil has the potential to absorb 5-7% hydrogen (by volume) prior to exiting the seal area. Because the system consumes large amounts of hydrogen, it is therefore not feasible for use on larger generators.
It has also been proposed to employ a separate sealing oil circuit in equilibrium with the generator atmosphere. This method is prevalent in today's hydrogen cooled generators. Such systems maintain high purity by utilizing two separate oil systems, one for untreated air and one for the treated hydrogen. However, employing two separate systems increases cost and operational difficulty.
One method of reducing impurities involves simply minimizing oil flow into the generator. This can be accomplished by reducing operating clearances of the seal ring and oil pressures. This method has resulted in excessive oil temperatures which has lead to thermal instability of the seal ring. Thus, reduction of clearances has not proven a viable option to maintain high hydrogen purity.
Still another suggested method of reducing impurities involves continuously purging all hydrogen from the system and replacing the hydrogen with fresh hydrogen from exterior to the system. Such systems are expensive to operate due to the costs associated with providing a constant supply of new hydrogen.
As an alternative to purging all hydrogen from the generator, it has been proposed to filter the hydrogen and recirculate the filtered hydrogen into the generator. U.S.
U.S. Pat. No. 4,531,070 ('070 Patent) entitled "Turbine Generator Hydrogen Filtration System," which is assigned to the assignee of the present invention and the contents of which are hereby incorporated by reference in their entirety, discloses such a system. The '070 patent discloses a filtration mechanism directed toward removing oil, gas-born particulates and gaseous contaminates such as styrene and anomine. The filtering mechanism disclosed in the '070 Patent comprises the following items: an air filter for removing solid particles one micron and larger from the gaseous stream; a coalescing oil filter tank which operates to coalesce very small particles of oil into larger particles or droplets; and a gaseous contaminant removing means for removing the oil droplets as well as gaseous contaminates such as styrene and anomine from the hydrogen stream. Thus, the filtering mechanism of the '070 Patent comprises three different filtering devices directed at removing oil, gas-borne particles, and gases such as styrene and anomine.
Although the filter of the '070 Patent is effective at removing most oil, air-borne particles, as well as styrene and anomine, it does not operate to remove air from the hydrogen stream. Air has the same deleterious effects as other contaminants and therefore optimally should be removed from the hydrogen stream. Further, the filtering mechanism of the '070 Patent comprises three different devices and therefore represents an inefficient filtering method.
Additionally, the coalescing filter of the '070 Patent cannot maintain adequate purity levels without employing additional treatment systems such as the vacuum system or separate sealing oil circuit described above.
It is therefore desirable to provide a more compact and efficient system that filters not only oil, particulates, and various gases, but also removes air from the hydrogen stream without the use of additional treatment systems.